Peak Identification - Tip 307

OBJECTIVE or GOAL

Match unknown peaks to the known peaks listed in the Component table of the Processing Method.

ENVIRONMENT

• Empower
• Tip of the Week #307

PROCEDURE

In the first tip in this series, we start with the process of matching unknown peaks to the known peaks listed in the Component table of the Processing Method.
Terminology in this tip: a "component" is a known peak that comes from the chromatogram used to develop the Processing Method, typically a standard. A "peak" is an unknown peak in a sample chromatogram that we want to identify. 

STEP 1
To identify an unknown peak, Empower does the following:

• It uses the time region defined by the retention time plus/minus the retention time window listed on the Component tab of the Processing Method, together with the selection in the Peak Match field (Figure 1).
• It matches peaks inside the retention time window of the components by calculating the difference between each unknown peak’s retention time and the component retention time.
• It uses the difference to choose the unknown peak that most closely matches the component peak. 

STEP 2
There is a hierarchy to how Empower matches peaks. Empower matches each component to the unknown peaks in its retention time window. If a peak matches multiple components, Empower determines the most appropriate component for that peak. The hierarchy is first by position, second by size, and third by retention time.

STEP 3
If the Match Type is set to either "Closest" or "Closest Negative", Empower calculates the absolute value of the component’s retention time minus the unknown peak’s retention time (Figure 2). 

STEP 4
A match is considered perfect if one of the following conditions exist:

• There is 0.0 difference between the retention time of the peak and the component.
• A peak is in the first, second, third, fourth, fifth, or last position in the retention time window that corresponds to the components match type.
• The size of an unknown peak relative to other peaks in the retention time window corresponds to the components match type (i.e., greatest area or height, least area or height, or greatest width [for GPCV data only]).

STEP 5
A component may match multiple unknown peaks, or an unknown peak may match multiple components. There are three possible outcomes of the initial matching process:

• A single peak matches a single component – When the retention time windows of the components do not overlap and one unknown peak is found in each window, the peak matching process is simple.
• Multiple peaks match a single component – If two peaks match a single component, Empower chooses the peak with the smallest match difference. The second peak is then matched to its next closest component. If two peaks equally match a single component, Empower does not match the component to either peak. 
• A single peak matches multiple components – If a peak matches multiple components, Empower picks the component with the smallest difference among the possible matches. If two components have equal differences for a peak, a choice cannot be made, and the unknown peak remains unmatched. In this case, Empower will list the component's Peak Type as Missing and the unmatched peak will display Q04 in the Peak Codes field in the Peaks table in Review (Figure 3).

ADDITIONAL INFORMATION

This can be done with either the Pro or QuickStart interface.